Defining Foliation in Metamorphic Rocks
Foliation refers to any penetrative planar (layer-like) fabric present in metamorphic rocks. It develops when a rock is subjected to directed pressure (differential stress) during metamorphism, causing its mineral grains to align perpendicularly to the direction of maximum stress. This alignment creates distinct parallel surfaces or bands within the rock.
How Foliation Forms
The formation of foliation involves several processes, including the rotation of existing platy or elongate minerals (like mica), the flattening of original grains, and the recrystallization or growth of new minerals in preferred orientations. The intensity of foliation depends on the magnitude of the differential stress, the temperature, and the abundance of platy or elongate minerals within the parent rock.
Common Types of Foliated Rocks
Examples of foliated metamorphic rocks include slate, phyllite, schist, and gneiss. Slate exhibits a very fine, planar foliation called slaty cleavage, allowing it to split into thin sheets. Phyllite has a slightly coarser, wavy foliation with a characteristic silky sheen. Schist shows prominent layers of visible platy minerals (schistosity), while gneiss displays distinct alternating bands of light-colored (felsic) and dark-colored (mafic) minerals, known as gneissic banding.
Significance of Foliation in Geological Studies
Foliation is a crucial indicator in geology, providing insights into the deformational history and conditions of rock formation. Geologists analyze foliation patterns to determine the orientation of past stress fields, reconstruct ancient mountain-building events (orogenies), and understand the tectonic forces that shaped Earth's crust. It helps classify metamorphic rocks and interpret their metamorphic grade.